The radio frequency (RF) spectrum is a limited commodity. Only a small portion of the spectrum can be assigned to each communications industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum. Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. The CDMA wireless communications system spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. A signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz.
All of the wireless access terminals, including both mobile stations and fixed terminals, that communicate in a CDMA system transmit on the same frequency. Therefore, in order for the base station to identify the wireless access terminals, each wireless access terminal is assigned a unique pseudo-random (PN) long spreading code that identifies that particular wireless access terminal to the wireless network. Typically, each long code is generated using the electronic serial number (ESN) of each mobile station or fixed terminal. The ESN for each wireless access terminal is unique to that wireless access terminal.
In some CDMA wireless networks, during the transmission of user data from a wireless access terminal to a base station (i.e., reverse channel traffic), the user data are grouped into 20 millisecond (msec.) frames. All user data transmitted on the reverse channel are convolutionally encoded and block interleaved to form a baseband signal. The baseband signal may then be modulated by an M-ary orthogonal modulation in which each N-bit data sequence or symbol is replaced by an orthogonal modulation code sequence of length M=2N. The M-ary modulated signal is then spread using a long code based on the ESN data and then separated into an in-phase (I) component and a quadrature (Q) component prior to quadrature modulation of an RF carrier and transmission.
Next, the I-component is modulated by a zero-offset short pseudo-random noise (I-PN) binary code sequence. The Q-component is modulated by a zero-offset short pseudo-random noise (Q-PN) binary code sequence. In an alternate embodiment, the quadrature binary sequence may be offset by one-half of a binary chip time. Those skilled in the art will recognize that the in-phase component and the quadrature component are used for quadrature phase shift keying (QPSK) modulation of an RF carrier prior to transmission. Those skilled in the art will also recognize that the access terminal may use binary phase shift keying (BPSK) modulation, quadrature amplitude modulation (QAM) or, other digital modulation format for modulation of an RF carrier for transmission of the data signals prior to transmission.
In some systems, the in-phase (I) data and the quadrature (Q) data may be transmitted as binary data, wherein two signal amplitude levels are possible: +1 (i.e., Logic 1) or −1 (i.e., Logic 0). However, in many types of systems, more than two signal levels are used. For example, both I and Q may take on eight discrete signal amplitude levels, such as −4, −3, −2, −1, +1, +2, +3, and +4. Thus, baseband binary data may be grouped into three bit octets having values of 000 to 111. Each octet is then translated into one of the signal levels [−4, −3, −2, −1, +1, +2, +3, +4] and transmitted as a corresponding amplitude. Thus, the data pair (I,Q) may take on 64 possible values.
FIG. 4A illustrates (I,Q) constellation 400, which contains sixty-four (64) possible (I,Q) values represented in a (X,Y) Cartesian (rectangular) coordinate system, for I=−4, −3, −2, −1, +1, +2, +3, +4 and Q=−4, −3, −2, −1, +1, +2, +3, +4.
In a typical wireless CDMA system that utilizes non-coherent demodulation, the phase relationship between the transmitter and receiver is unknown. As a result, the (I,Q) constellation at the receiver is not optimally centered in each quadrant and may even rotate slowly.
FIG. 4B illustrates (I,Q) constellation 450, in which the 64 possible (I,Q) values shown in FIG. 4A are rotated by a phase rotation error angle φ.
To compensate for this and to provide better demodulation of the received I and Q signals, a method of tracking and correcting the phase difference between the transmitter and receiver must be used. Some implementations incorporate phase tracking algorithms in the demodulation modems. There are two primary conventional methods of performing phase correction:
1) A modem is used for phase tracking and demodulation. This solution may not be desirable for testing reasons, where investigation is focused on a single hardware module that does not contain a modem. Additionally, in a production test environment, dedicating unnecessary resources (such as modems) to test various modules may not be desirable; and
2) An operator manually adjusts phase through manipulation of the transmitter or of the receiving circuitry. This solution is quite tedious, requiring significant manual input for every testing cycle. Additionally, the solution does not reject drifting of the phase due to equipment changes or frequency variations. Using a manual method of input, the operator only makes estimations of the optimum settings to correct any phase rotation and, therefore, any testing may have unnecessary errors introduced due to a less than optimum placement of the I/Q constellation.
Therefore, there is a need in the art for wireless networks that provide improved apparatuses and methods for tracking and correcting the phase difference between the transmitter and receiver. In particular, there is a need for a simple and robust phase-tracking apparatus that does not require a modem or operator intervention. Moreover, it is desirable that such a simple, robust phase-tracking algorithm be implemented in digital logic.